1. Field of the Invention
The present invention relates to a transmission device suitable for use in transmitting data via metallic lines such as private branch lines or customer-owned lines, and to a line equalizer control method, an integrating circuit and a frequency shift circuit each used in the transmission device.
2. Description of the Related Art
Modems are generally used to transmit data via private branch lines or the like. Particularly, low-priced modems with high transmission rates have been strongly desired. High-speed modems having a transmission rate of, for example, about 1.5 Mbps higher than that of conventional data transmission modems have been required because image information includes a large volume of information.
FIG. 33 is a diagram showing the configuration of a general modem. The modem 280 shown in FIG. 33 exchanges data signals with an opposite modem via a trunk. The modem 280 includes a receiving section 281 which receives a data signal via a trunk and output it to a terminal or the like, and a transmission section 286 which transmits the data signal from the terminal to an opposite device via the trunk.
The transmission section 286 functionally includes a logic processing unit 286a, a roll-off filter (ROF) 286b, a modulator 286c, a digital/analog (D/A) converter 286d, and others. The receiving section 281 functionally includes an analog/digital (A/D) converter 281a, a line equalizer 281b, a demodulator 281c, a roll-off filter 281d, an automatic gain control unit (AGC) 281e, an automatic equalizer (EQL) 281f, a carrier detecting unit (CD) 281g, a timing extracting unit 281h, a clock signal generator 281i, and others.
As a hardware, the combination of the transmission section 286 and the receiving section 281 is formed of an A/D converter, a D/A converter, a MPU (Microprocessor Unit), and a DSP (Digital Signal Processor) which executes a digital signal process.
In such an arrangement, a training signal is transmitted to the opposite device before starting a data transmission, prior to data signal exchange between the modem 280 and an opposite modem. The so-called training process is performed based on the training signal to adjust the automatic gain control circuit (AGC) and the line equalizer (LEQ) or the like in the receiving modem. In this training process, a receive signal having an attenuated level and deteriorated frequency characteristic caused by the line characteristic is adjusted to a suitable condition.
Thereafter, in the transmission section 286 in the modem 280 shown in FIG. 33, a signal point is generated in a transmission data signal from, for example, a terminal through the process of the logic processing unit 286a. The roll-off filter 286b subjects the signal point to a waveform shaping process. The modulator 286c modulates the resultant signal point. Then the D/A converter 286d converts the modulated signal into an analog signal and then outputs it as a data signal.
In the receiving section 281 arranged in the modem 280, the A/D converter 281a converts analog receive data input from an opposite device via a trunk into a digital signal. Then the demodulator 281c demodulates the digital receive signal.
Thereafter, the roll-off filter 281d subjects the demodulated signal from the demodulator 281c to a waveform shaping process. Sequentially, the automatic gain control unit 281e controls automatically the gain of the receive signal from the rolloff filter 281d. The automatic equalizer 281f equalizes the signal from the automatic gain control unit 281e and then outputs the resultant signal to a receive terminal or the like.
In order to configure a communications system by connecting the above-mentioned modem to a metallic trunk such as a private branch line, it is needed to consider the frequency characteristic of a metallic trunk illustrated in FIG. 34. That is, the metallic trunk has the frequency characteristic of 1/.sqroot.f shown in FIG. 34. The metallic trunk tends to distort the magnitude of a signal, particularly to more attenuate high frequency component in comparison with low frequency component.
When a receiving-side modem receives a data signal transmitted via a metallic trunk with the above-mentioned characteristic, the frequency characteristic of the receive signal in the receiving-side modem becomes 1/.sqroot.f. In the receiving section 281 arranged in the modem 280 shown in FIG. 33, the line equalizer 281b compensates the frequency characteristic of a receive signal changed due to the above-mentioned trunk characteristic. The line equalizer 281b can be formed of, for example, the above-mentioned DSP.
FIG. 35 is a diagram showing an example of the characteristic of the line equalizer 281b which equalizes the 1/.sqroot.f frequency characteristic of the above-described receive signal. That is, the line equalizer 281b has a .sqroot.f characteristic to correct the frequency characteristic deterioration caused by the trunk shown in FIG. 34. This feature enables the frequency characteristic of a receive signal to be flattened.
As for the characteristic of the line equalizer 281b, it is needed to change the slope of the characteristic shown in FIG. 35 in terms of the condition of a trunk, or degree of distortion in the amplitude of a receive signal. It is desired that the line equalizer 281b can handle the characteristic ranging from a flat portion to a sharp portion. In order to accomplish such a demand, the conventional line equalizer has employed a second-degree high pass filter (HPF).
However, in the line equalizer 281b within the receiving section 281 arranged in the modem 280 shown in FIG. 33, it is needed to set the HPF tap coefficient to a very large value to realize flat characteristic by using a second-degree HPF.
In this case, since the fixed-point arithmetic operation cannot be adopted to the digital signal process as the line equalizer, the floating-point arithmetic operation is adopted thereto. That is, a DSP employing the floating-point arithmetic operation must be used for the line equalizer. However, using the DSP employing the floating-point arithmetic operation results in an increased manufacturing cost and decreased processing rate.
Hence, when a system is configured using a DSP employing the above-mentioned floating-point arithmetic operation, it becomes more important problem to deal with the high manufacturing cost and reduced processing rate, rather than realization of flat characteristic.
In order to perform a data transmission using the modem 280 shown in FIG. 33, a training signal is exchanged prior to starting the data transmission. Then the equalizer and the automatic gain control circuit are controlled based on the received training signal. For that reason, a considerable long period of time is needed for the training operation.
Specifically, in the communications system which includes plural modems connected in parallel on a receiving side to broadcast data from a transmission-side modem, the time for training is needed, so that it is impossible to start transmitting data immediately after trunk connection.
In recent years, it has been desired to make the period taken to start data transmission as short as possible. However, as described above, the problem is that training which is performed with receiving-side modems before the starting of data transmission prolongs the data transmission start time.
Moreover, in order to respond to a current desire for increasing the processing rate of a modem, it is desired to reduce the signal processing function of the DSP or the like to the utmost, with the signal processing function of a conventional modem maintained.